1. Field of the Invention
The present invention relates to a drive unit. More specifically, it relates to a drive unit capable of moving a measuring tool for measuring surface texture of a workpiece such as surface roughness, waviness, and form along a surface of the workpiece.
2. Description of Related Art
An example of a drive unit capable of moving a measuring tool for measuring surface roughness, waviness, form etc. of a workpiece along the surface of the workpiece is known in U.S. Pat. No. 4765181 (related art 1).
The drive unit has a box-shaped frame, a sliding shaft provided in the frame in parallel with a moving direction of the measuring tool and holding the measuring tool, and a driver for moving the sliding shaft along an axial direction thereof. Both ends of the sliding shaft are supported by roller bearings in a sliding manner in the axial direction.
Since the sliding shaft of the drive unit is formed of a small diameter rod, flexure, for example, can be generated to the sliding shaft, thus impairing straightness of the sliding shaft. Accordingly, the linearity of the measuring tool secured to the sliding shaft can be impaired. Further, since the measuring tool is moved by displacing the sliding shaft in the axial direction thereof, the size of the frame has to be increased by the length of the sliding shaft itself and moving distance thereof, so that the moving distance of the measuring tool can be decreased relative to the length of the driving unit itself.
In order to overcome the above deficiencies, another drive unit disclosed in Japanese Patent Application No. Hei 11-190607 has been proposed (related art 2).
The drive unit has a box-shaped frame, a main shaft and a sub-shaft fixed to the frame, a slider slidably provided to the main shaft for holding the measuring tool and engaging to the sub-shaft, and a driver for moving the slider along the main shaft.
The driver does not move a rod member, such as the sliding shaft, as in the above-described related art 1 in the moving direction of the measuring tool, but moves the slider along the main shaft and the sub-shaft fixed to the frame, so that the slider (i.e. measuring tool) can be moved by the length of the shaft, thus increasing the moving distance of the measuring tool as compared to the driver of the related art 1. Accordingly, for achieving the same moving distance, the size of the related art 2 can be reduced when comparing the drive unit of the related art 1 with the related art 2.
However, since the drive unit of the related art 2 is arranged to move the slider holding the measuring tool along the main shaft while being engaged to the sub-shaft so that the slider does not rotate relative to the main shaft, structure thereof can be complicated on account of the need for providing the sub-shaft for preventing rotation of the slider relative to the main shaft, and the size and weight thereof can be increased.
Further, since the driver of the drive units of the related arts 1 and 2 is composed of a motor and a feeding threaded shaft provided serially relative to the shaft of the motor, the moving distance of the measuring tool (i.e. the length of the feeding threaded shaft) cannot be lengthened as compared to the size of the drive unit (total length of the motor and the feeding threaded shaft). In other words, for increasing the moving distance of the measuring tool, the size of the drive unit itself has to be increased.
An object of the present invention is to provide a drive unit capable of maintaining linearity of the moving direction of the measuring tool with a simple structure and capable of reducing size and weight thereof.
A drive unit according to the present invention is for moving a measuring tool along a surface of a workpiece. The drive unit is characterized in having: a frame; a guide rail provided to the frame, the guide rail having at least two reference surfaces in parallel with a moving direction of the measuring tool, the reference surfaces mutually crossing at a predetermined angle; a slider slidable along the reference surfaces of the guide rail, the slide holding the measuring tool; a pre-load means for biasing the slider toward the reference surfaces of the guide rail; and a driver for moving the slider along the guide rail.
According to the present invention, the slider holding the measuring tool is moved without being rotated along a single guide rail having at least two reference surfaces parallel with the moving direction of the measuring tool, the reference surfaces retaining a predetermined angle with each other. Accordingly, since the cross section of the guide rail can be enlarged without increasing the size of the entire drive unit, the size and weight of the drive unit can be reduced while securing high rigidity as compared to a conventional arrangement having the two shafts. Therefore, the linearity of the moving direction of the slider and the measuring tool being moved along the reference surfaces of the guide rail can be achieved with high accuracy.
Further, since the slider is always biased by the pre-load means toward the reference surfaces of the guide rail, the position of the slider can be maintained constant even when a clearance is formed between the slider and the guide rail. Accordingly, the position or orientation of the slider (i.e. the measuring tool) does not become unstable on account of the clearance of the sliding portion, and the movement of the measuring tool can be securely repeated.
In the present invention, the guide rail may preferably be formed in an approximately reverse U-shape and the slider is formed in an approximately reverse U-shape corresponding to the configuration of the guide rail.
According to the present invention, since the guide rail and the slider are formed in an approximately reverse U-shape, the weight of the guide rail and the slider can be reduced while maintaining high rigidity.
In the present invention, the pre-load means may preferably have a first pre-load means for biasing the slider to one of the reference surfaces of the guide rail and a second pre-load means for biasing the slider to the other one of the reference surfaces of the guide rail.
According to the present invention, since the first and the second pre-load means biasing the slider to the reference surfaces of the guide rail are provided to the approximately reverse U-shaped guide rail and slider, the slider can be biased toward the respective reference surfaces of the guide rail at an appropriate force. Further, when the two reference surfaces respectively extend perpendicularly and horizontally, the slider may preferably be biased toward the guide rail at relatively high pressure on the perpendicular reference surface and the slider may preferably be biased toward the guide rail at relatively low pressure on the horizontal reference surface considering gravity (weight) of the slider, so that the slider is biased toward the respective reference surfaces of the guide rail at an appropriate force. Therefore, slidability and linearity of the slider can be simultaneously attained.
According to the above arrangement, since the leaf spring fixed to the slider presses the slide pieces toward the guide rail for biasing the slider toward the guide rail, the slider can be securely biased to the guide rail with a simply-structured pre-load means.
Further, by using material having a small friction coefficient, such as TEFLON (trade name, i.e., polytetraflouroethylene), for the slide piece, no great friction force is caused between the guide rail and abutting surfaces of the slide piece even when the slider is biased toward the guide rail by the first and the second pre-load means, so that the slider can be smoothly slid along the guide rail.
According to the above arrangement, the motor is rotated and the rotation force of the motor is transmitted to the feeding threaded shaft through the connecting mechanism, so that the feed piece screwed to the feeding threaded shaft is advanced and retreated, thus moving the slider and the measuring tool. In the driver, since the shaft of the motor and the feeding threaded shaft are parallel, the feeding threaded shaft can be extended over the maximum available longitudinal length of the drive unit, thus securing a longer moving distance of the measuring tool than in the conventional arrangement. In other words, when the moving distance of the measuring tool is equal to the length of the feeding threaded shaft, the size of the drive unit itself can be reduced as compared to the conventional drive unit.
In the present invention, both ends of the guide rail may preferably be integrally fixed to both ends of the frame.
According to the present invention, since both ends of the guide rail are integrally fixed to frame, the size of the guide rail can be increased relative to the size of the drive unit, thus further enhancing the rigidity of the frame. Therefore, when the drive unit itself is supported by a cantilever construction, since the frame and the guide rail do not distort, the linearity of the moving direction of the measuring tool can be maintained.